For instance: the command
“type c:\anyfile.exe > c:\winnt\system32\calc.exe:anyfile.exe”
will fork the common windows calculator program with an ADS “anyfile.exe.”
Alarmingly files with an ADS are almost impossible to detect using native file browsing techniques like command line or windows explorer. In our example, the file size of calc.exe will show as the original size of 90k regardless of the size of the ADS anyfile.exe. The only indication that the file was changed is the modification time stamp, which can be relatively innocuous.
Once injected, the ADS can be executed by using traditional commands like type, or start or be scripted inside typical scripting languages like VB or Perl. When launched, the ADS executable will appear to run as the original file - looking undetectable to process viewers like Windows Task Manager. Using this method, it is not only possible to hide a file, but to also hide the execution of an illegitimate process.
Unfortunately, it is virtually impossible to natively protect your system against ADS hidden files if you use NTFS. The use of Alternate Data Streams is not a feature that can be disabled and currently there is no way to limit this capability against files that the user already has access to. Freeware programs like lads.exe by Frank Heyne (www.heysoft.de) and crucialADS by CrucialSecurity can be used to manually audit your files for the presence of Alternate Data Streams. Alternatively, the action of moving a file into another file system that doesn’t support ADS will automatically destroy any Alternate Data Streams.
Ultimately only a third party file checksum application can effectively maintain the integrity of an NTFS partition against unauthorized Alternate Data Streams. Recently dubbed as host based “Intrusion Prevention Systems” or “Intrusion Detection Systems”, third party security applications like eTrust Access Control from Computer Associates have been used for years in high-end government networks to verify the integrity of files used in the most secure environments. In addition to a heightened level of auditing and access control, these applications typically create an MD5 hashed database of file checksums that are used to validate a file’s trustworthiness. File injection techniques like Alternate Data Streams trigger an action by which the file is deemed untrusted and therefore prevented from executing or better yet, prevented from being changed in the first place.
In order to fully understand the implications of alternate data streams, the following walkthrough the creation and execution of an ADS using standard Windows 2000 programs on an NTFS 5.0 partition.
Figure 1 shows the executable file for the standard windows program calculator, calc.exe, with the original size of 90KB and a date modified time stamp of 7/26/2000.
We then append an alternate data stream to calc.exe with another standard windows program, notepad.exe as shown in
Figure 3 shows that while notepad.exe is 50KB, the file size of calc.exe has not changed from the original 90KB. We do see however that the date modified time stamp has changed.
we execute the new ADS notepad.exe using the standard command start.
On our desktop, the program notepad is executed however, an examination of the Windows Task Manager shows the original file name calc.exe. (Figure 5).
Figure 5
Summary
Ultimately, the mere availability of Alternate Data Streams in NTFS is quite disconcerting and their usefulness suspect but in the end, the security features of NTFS far outweigh this potentially dangerous vulnerability. With knowledge and due diligence administrators can take actions to prevent and detect unauthorized use of ADS and in the end protect themselves adequately.
